1. Technical Field
The present invention relates to flexible circuit boards capable of directly supporting unpackaged IC chips and the process of producing the same. More particularly, the present invention related to a method for the fabrication of a to IC-mounting flexible circuit board comprising the steps of forming holes in the insulating base of a flexible circuit board, preferably made by using a non-adhesive flexible electrically-conductive laminated board, and forming protruding conductive junction portions on the board by plating the holes so that the protruding junction portions may be employed to establish electrical connections to the pads of the unpackaged, bare, i.e., IC-chips.
2. Prior Art
There are known methods of mounting unpackaged IC chips on circuit boards using junction bumps, including the tab method wherein, as shown in FIGS. 10A and 10B, bumps 3 formed on respective bonding pads 2 of a bare IC chip 1 or bumps 6 formed on respective lead fingers 5 on chip carrier tapes 4 are employed for providing connection by means of heat fusion or ultrasonic bonding, and the proposed flip chip method wherein, as shown in FIG. 11, bumps 3 formed on a bare IC chip 1 are connected to respective conductive patterns 8 of a circuit board 7 by soldering. In the case of the flip chip method, it is common to provide solder-flow preventing dams on the respective conductive patterns 8 as shown in FIG. 11.
In the case of the tab method of FIG. 10A, however, a number of steps are required to form the bumps 3 on the bare IC chip 1, which will result in cost increase. Alternatively, although it has been attempted to adopt a method of transferring the bump 6 to the leading end of each lead finger 5 as shown in FIG. 10B, this method necessitates not only a transfer bonding step but also the provision of special jigs for making the bump. Moreover, there is still a limit in increasing its structural density. As a support is not provided for the lead finger 5 in both methods described above, its mechanical strength is insufficient and as the lead finger 5 is provided with no insulating material, the bonding pads 2 cannot be disposed zigzag. In addition, because of the projecting structure of the lead finger 5, it is prone to deformation and this makes it difficult to maintain the positional accuracy in the directions of X, Y and Z.
In the case of the flip chip method of FIG. 11, on the other hand, the formation of the bumps 3 on the IC bare chip 1 requires a number of steps, which also results in cost increase likewise. Moreover, the necessity of forming the solder-flow preventive dam 9 in the vicinity of each pad of the circuit board 7 also tends to increase the cost further and there is still a limit in increasing its structural density.